Relative age dating of rocks
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.For purposes of relative dating this principle is used to identify faults and erosional features within the rock record.Then, by applying the Principle of Cross-Cutting we are able to relatively date those processes.The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across.Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.There are two basic approaches: relative age dating, and absolute age dating.
Once a rock is lithified no other material can be incorporated within its internal structure.This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface.Therefore newer sediment is continually deposited on top of previously deposited or older sediment.With absolute age dating, you get a real age in actual years.It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.